The invention relates to a control device for operating a wiper motor (10), in particular, for a motor vehicle.
According to German published patent application DE OS 28 47 765, a circuit arrangement to protect a drive motor from overload has already been disclosed, wherein the measuring circuit comprises a memory circuit having a storage element that can be charged with each detection of a motor overload current and that can be discharged again during the time period between successive detections of respectively separately occurring motor overload currents. Furthermore, an evaluating device is also connected to the storage element with the evaluating device only emitting an output signal that shuts down the motor when the voltage at the cited storage element exceeds a predetermined trigger threshold.
Short-time motor overloads only result in a shutdown of the motor when these overloads correspond to a dangerous overload of the motor over the respectively considered time period. This means that short-time overloads of the motor must be stored over a time period so as to interrupt the motor circuit when a predetermined threshold value is reached.
During this process, the voltage, which is measured at a shunt resistor and which corresponds to the motor current, is supplied to a comparator, which emits an output voltage on the output side corresponding to the difference between the two voltages supplied on the input side when the threshold voltage is exceeded. A storage capacitor is charged with this output voltage. The charging time constant of the storage capacitor is selected to be considerably smaller than the discharging time constant. In this manner, corresponding charges are accumulated on the storage capacitor. If the voltage that is present at the capacitor exceeds a further threshold value, the electric circuit of the motor is interrupted.
The drawback of this circuit arrangement is that the short-time overloads that follow one another are accumulated. This results in a so-called build-up effect which is realized by the selection of the ratio of charge time constant and discharge time constant of the storage capacitor.
A further drawback is that the storage element only responds when a motor overload current is detected, i.e., the threshold is not exceeded until an overload occurs.
In particular, this circuit arrangement cannot be used for a wiper system having a current path which characterizes the wiping motion of wipers because the current path is subject to great variations and the admissible overload would be accumulated in an undesirable manner.